In a multiple access wireless communications network, a plurality of users/transmitters communicate simultaneously with a single receiver in a given wireless spectrum. Given the shared nature of the wireless spectrum, however, signals transmitted by different users may collide at the receiver causing a loss of transmitted information and of network resources. This problem is known as multiple access interference (MAI) and is a main factor that limits the capacity and performance of a multiple access network.
In some cases, to remedy the MAI problem, the available spectrum in a network is multiplexed in time and/or frequency among available users/transmitters in the network. Each user may then receive a dedicated access time and/or frequency multiplexed channel to communicate with the receiver. Signals transmitted by different users in the network are then said to be orthogonal (in time and/or frequency), and, as a result, cause no interference to each other.
While significantly reducing potential MAI in a network, time and/or frequency multiplexing, on the other hand, often result in a far from optimal usage of the capacity of a network. For example, in a time division multiple access (TDMA) system, unless each user always has information ready to transmit during its allocated time slot, time opportunities to communicate with the receiver will be wasted resulting in a degraded throughput performance of the system.
In recent years, spread-spectrum-based code division multiple access (CDMA) has taken a greater role in multiple access networks. By using a unique spreading code for each user, CDMA eliminates the need for orthogonality in time and/or frequency among signals in a network. Typically, the unique spreading codes ensure low signal cross-correlation over a wideband, and allow for CDMA signals to be successfully decoded at a receiver in the presence of permissible interference.
Nonetheless, a low signal cross-correlation, as required by CDMA, is very challenging to maintain in a wireless spectrum due to unpredictable channel conditions. In fact, the random time offsets between signals that occur in a wireless spectrum make it difficult to ensure that CDMA signals are completely orthogonal. This may result in reduction of network capacity and throughput. Furthermore, since CDMA signals are typically spread over a very wide bandwidth, possibly the entire network's spectrum, transmit power level considerations impose further limitations on the capacity of CDMA systems.
In the last few years, however, multi-user detection (MUD) has received considerable attention as an area of research holding the key to improving the capacity and to alleviating some technical requirements of CDMA systems. Many algorithms for performing multi-user detection have been put forth. These range from the high-complexity optimum detectors to many forms of sub-optimum lower complexity detectors.
While a good number of the current solutions have shown to be too complex for actual implementation, a common limitation of these solutions is that they rely on spread-spectrum modulation to keep the cross-correlation between signals low. Consequently, this limits the applicability of these techniques to spread spectrum systems such as CDMA.
What is needed therefore is a multi-user detection technique having low implementation complexity and that supports detection for highly correlated signals. In other words, a multi-user detector is needed for both spread spectrum and non-spread spectrum systems that enables multiple access communication in a code, time, or frequency-multiplexed system.